1. Field of the Invention
The present invention relates to a method and apparatus for resetting a photo-electric converter, and more particularly to resetting method and apparatus of a photo-electric converter which effects a first reset function to reset a charge stored in a control electrode to a first potential, and a second reset function to cause a main electrode of an output circuit to assume a second potential after the first reset function to reset the residual charge.
2. Related Background Art
A sensor used in an image reader such as a facsimile machine uses bipolar transistors as shown in Japanese Laid-Open Patent Application No. 60-12764.
FIG. 2 shows an equivalent circuit of an element of the sensor.
As shown in FIG. 2, the sensor comprises an NPN bipolar transistor T having a base thereof connected to a P-channel MOS transistor M.sub.RES, and an emitter thereof connected to an N-channel MOS transistor M.sub.VRS. A charge is stored in the base by light irradiation and an electrical signal representing the stored charge is produced at the emitter.
When a light is applied to the sensor while the base floats at a predetermined potential, a charge is stored and the base potential rises. As the base potential rises, an electrical signal is read out from the emitter.
A method for resetting the sensor after reading is now explained.
In FIG. 3, (A) and (B) show timing charts for the resetting method of a prior art sensor, (C) and (D) show changes in base and emitter potential levels for low light exposure, and (E) and (F) show changes in the base and emitter potential levels for high light exposure.
When a pulse .phi..sub.RES is at a negative potential for a period T.sub.1 under low light exposure as shown in FIG. 3, (A), (B), (C) and (D), the P-channel MOS transistor M.sub.RES conducts and the base of the NPN bipolar transistor T is reset to a potential V.sub.BG. (This is called a perfect reset).
When the pulse .phi..sub.VRS is at a high level for a period T.sub.2, the N channel MOS transistor M.sub.VRS conducts and the emitter of the NPN bipolar transistor T is clamped to a potential V.sub.VR (V.sub.BG &gt;V.sub.VR).
When the emitter potential V.sub.EL is clamped to V.sub.VR, the base-emitter is forward biased and the base potential V.sub.BL converges to a fixed potential V.sub.K. (This is called a transient reset). When the pulse thereafter falls, the N-channel MOS transistor M.sub.VRS is turned off and the base shifts to a store status. (This is called a store operation). In FIG. 3(C), SL represents a signal component.
On the other hand, under high light exposure, both the base and the emitter are near saturation potentials as shown in FIG. 3, (A), (B), (E) and (F). If the pulse .phi..sub.RES is at the negative potential during the period T.sub.1 so that the perfect reset is effected, the base potential V.sub.BH is set to a potential V.sub.BG and the base-collector and the base-emitter are reverse biased.
When the pulse .phi..sub.VRS next assumes the high level during the period T.sub.2, the base potential V.sub.BH is pulled to the emitter potential through a capacitance C.sub.BE (base-emitter capacitance) at the moment of the change of the emitter potential V.sub.EH to V.sub.VR so that it changes to a potential V.sub.K ' which is lower than V.sub.BG. As a result, the base-emitter is not forward biased and the intended transient reset is not effected, and the base starts the store operation from the lower potential V.sub.K ' than V.sub.K. Since the store operation starts from the lower potential V.sub.K ' under the high light exposure rather than V.sub.K as under the lower light exposure, the signal level of the signal drops by (V.sub.K -V.sub.K '). This means that a real signal component S.sub.H2 is read as an apparent signal component S.sub.H1. As a result, the linearity of the photo-electric conversion characteristic in the high irradiation range is deteriorated.